1. The Field of the Invention
The present invention relates to optical sub-components for use in optical systems. or networks. More particularly, the present invention relates to systems and devices to reduce internal reflections of an optical signal in one or more optical sub-assemblies.
2. The Relevant Technology
Optical networks are widely used to communicate data over short and long distances in various networks, including telecommunications and data networks. Optical networks using optical fibers have become a preferred way of transmitting data due to the high bandwidth-inherent in optical networks, as well as the decreasing cost of components of such networks. In optical networks, data is encoded in optical signals, which are transmitted over optical fiber(s) between nodes in the network. These optical signals propagate along an optical fiber as the signal is internally reflected within a core of the optical fiber. The materials forming the core and the cladding of the optical fiber have different refractive indexes. Optical signals incident upon the interface between the core and the cladding is internally reflected rather than being refracted in accordance with the rule that light incident upon an interface between materials having different refractive indexes is reflected or refracted at such an interface. The amount of light being reflected or refracted and the directions of reflection or refraction depend upon the angle of incidence with the interface and the refractive indexes of the media across the interface.
Optical signals delivered along these optical fibers are generated using a laser included in a transmitter optical sub-assembly (TOSA) of a transceiver positioned at a node in the network. The transceiver converts electrical signals to optical signals. Optical signals generated by the transceiver and propagated by the optical fiber are received and detected by a photodetector that can be included in a receiver optical sub-assembly (ROSA) of another transceiver positioned at another node in the network. Because bi-directional communication is typically desired and is easily achieved in optical networks, transceivers generally include both a TOSA and a ROSA.
As mentioned above, the amount of light being reflected or refracted and the directions of reflection or refraction depend on the angle of incidence with the interface and the refractive indexes of the media at the interface. For example, approximately 4% of the light traveling from glass into air and approaching an interface between the glass and the air at a normal direction is reflected backward into the glass along the same normal direction of incidence. The remaining 96% of the light passes through the interface and proceeds into the air. In the case of an optical fiber terminated into air, as is typically the case with current TOSAs and ROSAs, the light reflected from a fiber end propagates back into the fiber and travels in the opposite direction of the incoming light. Light propagating in opposite directions in a fiber can cause interference with other signals. If the fiber is connecting two optical transceivers, as in a standard communication link with a transmitter and a receiver at opposite ends, this optical interference can degrade the quality of the signal being transmitted. In particular, when significant reflections exist at both ends of an optical link, an optical cavity is formed with a net transmission that varies with wavelength. Small dynamic changes in a laser's wavelength (also known as chirp) can be converted to amplitude modulations by the cavity thus formed. These amplitude modulations can significantly impair the overall optical transmission link adding noise to the optical zero and one levels and reducing minimum difference between these values, which in turn introduces an optical power penalty to a link.
Additionally, reflections returning to the laser diode transmitter can induce a number of significant impairments in its output such as deleterious changes in the optical spectrum and large increases in the relative intensity noise (RIN) of its output.
Currently the return loss, i.e. the loss of signal strength resulting from such reflections or the loss in the reflection relative to the incident signal, is typically in the range of about −14.4 dB. Various optical networks have different requirements for this return loss. For example, the SONET specification defines the maximum allowable return loss as −27 dB. To meet these requirements, conventional optical sub-assemblies (OSAs) use a fiber stub that abuts the terminal end of the optical fiber. The terminal end of the fiber stub positioned away from the fiber end generally has a facet polished at an angle of about 6° to about 10° from being perpendicular to the direction of travel of the optical signal. Because the terminal end of the fiber stub has an angled facet, much of the optical signal internally reflected at the glass/air interface at the fiber stub terminal end is not transmitted back into the optical fiber, thus resulting in a reduction in the amount of light reflected at the interface and hence improving the return loss. Alternatively, the terminal end of the fiber stub can be coated with a set of dielectric layers that reduce reflections.
It should be noted that elimination of reflections from fiber facets is a necessary but not sufficient means to achieve high return loss. An optical subassembly must be designed so that reflections originating in other parts of the subassembly are not coupled back into the incoming fiber. It is presumed in all the examples of prior art and in the new invention that such measures have been taken to achieve an overall return loss target once the fiber reflection is reduced sufficiently.
While the use of such fiber stubs can successfully bring the return loss to the level required by the SONET and other specifications, fiber stubs introduce other problems into the transceiver design. For example, optical components, including transceivers, are becoming smaller and more compact. However, because of fabrication and other considerations, a fiber stub generally requires at least 2 mm of length in the transceiver. Additionally, the fiber stub increases the cost of the optical transceiver. Lastly, in the case of an angled fiber stub, there is a variation in the optical power coupled from a laser into the fiber that depends on (i) the degree of offset of the laser with respect to the fiber axis and (ii) the orientation of the fiber stub angled facet. Such variations result in a broadening of the distribution of output optical power.
There is, therefore, a need for methods, systems and devices that reduce reflection in optical components, improve the return loss, and limit the size of the transceiver.